Adding Label Support to Docker 1.6

Over the last few months our team, with the help of Daniel Walsh (@rhatdan) from Red Hat and many other community members, have worked to add support for labels in Docker 1.6. Labels allow users to attach arbitrary key value metadata to Docker images and containers. This feature, while very simple in concept, gives us the opportunity to add many powerful features to Rancher, and will benefit everyone in the Docker ecosystem.

If you understand how environment variables work in Docker, you will see that container labels function very much the same way. Labels are similar to environment variables in that they are key value pairs attached to an image or a container. Labels are different from environment variable in that they are not visible to the running application and they can also be used for fast look-up of images and containers.

Container and Image Labels

To apply a label to an image just use the LABEL command in your Dockerfile. For example

FROM busybox
LABEL com.example.foo=bar

docker build -t test .

Once you have your image with a label, you can then search for your image using

docker images -f label=com.example.foo=bar

If you create a container from that image that label will be applied to the running container.

Using Labels with Rancher

At Rancher Labs we plan to use labels extensively. For example, Rancher, our orchestration system, provides the ability to deploy a container-based, auto-scaling load balancer. In order to load balance a workload, you must specify the targets you want to direct traffic towards. There are many ways to do that, but we wanted to find the simplest and most flexible way. Also, we’ve made it a priority to always support a Docker native interface. If we have to come up with our own Load Balancing API, that is not as desirable as using an approach based on Docker’s interface. With labels, that is now possible.

When a user starts a container all they have to do is add the label “io.rancher.service.provides=foo”. There is then an existing load balancer configured in Rancher that will see that container with that label and then add it to the existing Load Balancer. This means you can use various means to add targets to load balancers. For example a configuration management tool like Puppet or Chef or using Amazon’s auto scaling services. Through the power of labels we can attach additional metadata to containers that can be used to add more services to the container.

I hope you’ll find this feature as useful as we think it will be. If you’d like more information on using Rancher to manage containers, please join our next online meetup, where we’ll be demonstrating some of these capabilities